(1) Field of the Invention
The present invention relates to a process for firing ceramic shaped bodies and firing tools used therefor.
(2) Related Art Statement
Ceramic shaped bodies, which are prepared by mixing a ceramic raw material with an appropriate firing aid and then extruding a ceramic body thus obtained, are conventionally fired as follows:
For example, as shown in FIG. 8, a plurality of ceramic shaped bodies 4 are sidewisely inserted and put onto shelves 3 on a truck 2 inside a tunnel kiln (or a periodic kiln), and the shaped bodies are fired by heating a kiln-inside atmosphere to a given temperature by means of burners extending through side walls 5 of the kiln 1. Such a firing kiln is generally called "side firing system".
As shown in FIG. 9, an under firing system in which burners 6 are located at upper and lower portions of side walls of a kiln has also been used as the firing kiln.
However, in the above-mentioned ceramic firing processes, since ceramic shaped bodies are exposed directly to a combustion gas jet stream, the shaped bodies are partially melted away or locally thermally deformed. For instance, when ceramic shaped bodies are honeycomb structural bodies made of cordierite, they are generally fired at about 1,400.degree. C. On the other hand, since the melting point of the honeycomb structural body is about 1,450.degree. C., they are partially melted away at a place where a combustion gas jet stream directly contacts the body. That is, in the case of the side firing system in FIG. 8, combustion gas jet streams from the burners as shown by arrows directly contact the ceramic shaped bodies 4 through openings provided on sides of the shelves. In the under firing system in FIG. 9, combustion gas jet streams pass above or under the shelves 3, are reflected by the side walls 5, and contact the ceramic shaped bodies through openings defined by the shelves 3. In both the side firing system and the under firing system, the combustion gas jet stream passes through a gap between the shelves. Thereby, ceramic shaped bodies arranged near the gap are influenced, or combustion gas jet streams are impinged upon supports provided on the truck during movement of the truck and the gas jet stream becomes a turbulent so that the gas jet stream directly contacts the ceramic shaped bodies. On the other hand, with respect to shaped bodies even inside the same shelf, there are naturally variations between products which are obtained while contacting directly with the combustion gas jet streams and other shaped bodies.
In particular, when the ceramic shaped body is of a honeycomb structure, a number of through holes are defined by thin walls. Thus, not only the melt away and thermal deformation of the outer peripheral portion of the honeycomb structural body occur, but also shrinkage deformation or cuts of the thin walls due to difference in temperature between the inside and outside of the honeycomb structure bodies.